1. Field of the Invention
The present invention relates to cementitious construction sheets, such as gypsum wallboard and more particularly refers to a new method for forming paper-free glass fiber-reinforced cementitious sheets such as gypsum and utilizing the sheets with a standard cementitious slurry core to form paper-free wallboard, and to the products formed thereby.
2. Description of the Prior Art
Wall panels or wallboards made of rehydrated stucco conventionally comprise a gypsum core of uniform density sandwiched between two paper cover sheets. Such panels can be mass produced and erected so inexpensively that they have largely replaced prior building techniques using wood panels or plaster. As the usage of such wallboard has expanded, however, specialty uses such as walls in high-rise office buildings and apartments have placed a premium on certain properties. Specifically, shaft walls used, for example, as elevator shafts, air return shafts, and stairwells are subject in some instances to very strict fire regulations. Thus there is a trend in municipal fire codes towards requiring a 0-0-0 fire rating for the exposed surface of elevator shafts, that is, having zero flame spread, zero smoke, and zero toxic gas generation. It has not been possible to achieve such ratings as long as paper-covered wallboard is used, due to the combustibility or at least the smoke-generation capability, of the paper cover sheets. Such paper cover sheets are further troublesome in that they appreciably delay the drying time of the board during its manufacture.
A further problem characteristic of certain elevator shafts is that wind loading causes constant flexing of the wallboard. Thus, when used in such walls, the wallboard must have good flexural strength--a physical property not exhibited by rehydrated stucco alone due to its low modulus of rupture.
Some presently manufactured wallboard does include various ingredients which impart fire resistance to the board. For example, glass fibers on the order of one-half inch in length have been incorporated throughout the core of paper-covered gypsum wallboard used to line elevator shaft walls, on a weight percent basis of about 0.25% of the weight of the board. However, such fibers are not long enough to contribute significantly to the flexural strength of the board, as the concentration is insufficient, and at that length, the fibers' pull-out strength is insufficient.
Numerous methods have been developed through the years to combine glass fibers and gypsum in order to produce various articles such as reinforced gypsum wallboard. It was early realized that in order to develop good strength properties the glass fibers must be evenly dispersed and a gypsum matrix must be used which has high strength.
When glass fibers and gypsum are mixed in the form of an aqueous slurry, the length and amount of fibers which may be added are limited in order to prevent balling of the fiber during mixing. Excess water is also required to make the slurry sufficiently fluid to be formed into the desired article. The use of excess water reduces the strength of the gypsum matrix. This is disclosed in British Pat. No. 1,204,541. In order to remove excess water, a means of applying suction and pressure to the formed board has been developed. This process is cumbersome and costly and not well adapted to high speed production.
A similar approach is illustrated in New Zealand Pat. No. 155,679, which teaches a gypsum panel constructed with glass fibers of various lengths, dispersed generally throughout the rehydrated stucco. Such a construction has eliminated the need for a paper cover sheet. However, the process of making such panels is difficult, time-consuming, and involves the use of a large proportion of glass fibers, inasmuch as they are distributed more or less uniformly throughout the board or panel.
Another prior art process for producing glass fiber-containing gypsum board involves co-spraying discontinuous glass fibers and gypsum in an aqueous slurry onto a moving belt. Special low water demand plaster is used, or suction is used to remove the excess water. The major disadvantages of this process are that the fiber strands are not dispersed into individual filaments of which they are formed, thus reducing the efficiency of the fiber as a reinforcing agent, and that the fibers are not sufficiently mixed with the plaster.
Glass fiber mats have also been used to reinforce gypsum. These mats may be in the form of continuous or discontinuous, random oriented fiber, or as woven mats. The mats are saturated with gypsum using various means and methods. One method is described in Canadian Pat. No. 993,773. The mats are fabricated with glass fiber strands each consisting of a plurality of glass fiber filaments. The gypsum slurry does not saturate the fiber strand and therefore the reinforcing efficiency of the fiber is reduced. Special low water demand gypsum such as alpha-calcium sulfate hemihydrate must be used to obtain high gypsum strength. Alternatively, excess water must be removed by means of suction.
In copending U.S. application Ser. No. 666,539, filed Mar. 15, 1976, and which is a continuation of U.S. Ser. No. 592,960 filed July 3, 1975, now abandoned, which is in turn a continuation of U.S. Ser. No. 415,038 also now abandoned, there is disclosed a reinforced gypsum board comprising a core of relatively low density being free of glass fibers, and provided on both surfaces thereof with skin layers formed of rehydrated gypsum having glass fibers dispersed therein. Preferably the outer layers are formed of gypsum having a higher density than the gypsum of the core. The preferred material for making the higher density outer layers is alpha calcium sulfate hemihydrate. Each outer layer is preferably bonded to the core by intercrystalline growth during the forming process. In forming the product, an aqueous stucco slurry containing glass fibers is deposited on a moving flat surface to form one outer layer, a low density aqueous slurry of calcium sulfate hemihydrate is deposited over the first outer layer to form a core, and a second outer layer of an aqueous slurry of calcium sulfate hemihydrate containing glass fibers is deposited over the core layer. No paper cover sheets are applied to the outer surfaces of the board. The resulting gypsum wallboard is relatively light since a low density gypsum is used for one core, and is very strong due to the glass fibers contained in the outer layers. A further advantage of the product is that there are no cover sheets to hinder the evaporation of excess water in the drying kiln, thereby reducing the processing time and reducing the cost of fuel for drying. The resulting board also has a high flexural strength. Moreover, due to the central plane of symmetry of the board the strength is the same regardless of the face on which it is measured. Because a relatively low density gypsum is used for the core and relatively high density gypsum is used for the thin outer skins, the increase in strength is achieved without a concommitant increase in overall weight. The product is stated to be excellent for use in elevator shaft wall and for use in elevator air return shafts and stairwells where the product is subject to very strict fire regulations, and must undergo considerable flexing due to wind loading.
In U.S. Pat. No. 3,682,670, there is disclosed a process for preparing fiber-containing plaster products wherein glass wool and/or rock wool is carded, plaster powder added to the fibers as they are carded to provide a dry composition, and the dry mixture is then introduced into an excess of water to form a slurry which is subsequently cast in the form of boards. However, this process is somewhat deficient in that a large excess of water is introduced which must be subsequently removed by drying. Additionally, the carding process does not produce a good uniform mix of the fibers and plaster.
In U.S. Pat. No. 1,862,318, a method is disclosed for producing plaster board containing cotton linters which comprises first depositing a layer of gypsum on a moving belt subsequently depositing the cotton linters thereover while carding, and finally sprinkling water over the layer thus formed and compressing the layer by rolling. In this method, because the gypsum and fibers are not premixed but, merely sprinkled onto the belt, a uniform layer is not produced.